Conventionally, there is known a system for generating hydrogen-rich reformed gas by the steam-reforming of a mixture of raw material gas including hydrocarbons such as methane, aliphatic alcohols such as methanol, or ethers such as dimethyl ether, with steam, (hereinafter referred to as the “raw material-steam mixture”), under the presence of a steam reforming catalyst. The hydrogen-rich reformed gas obtained from the reforming system is favorably used as the fuel of fuel cells. The reformer which is a main component of the reforming system is classified into the external heating type and the internal heating type in view of the mode of supplying heat necessary for the steam reforming reaction. The reaction formula of steam reforming when methane is used as the raw material gas is written as CH4+2H2O→CO2+4H2, where a preferable range of reforming reaction temperature is from 700° C. to 750° C.
The former external heating type heats externally the wall surface of the reformer by a combustion gas generated by a burner and the like, thereby supplying the heat necessary for the reforming reaction through the wall into the reaction chamber.
Internal heating type is a modified version of the external heating type, and constituted to have a partial oxidation reaction bed at the supply side (or the upstream side) of the raw material-steam mixture in the reformer. The heat generated in the partial oxidation reaction bed is used to heat the steam reforming bed located at the downstream side to the steam reforming temperature. The steam reforming is carried out in thus heated steam reforming catalyst bed to generate the hydrogen-rich reformed gas. The partial oxidation reaction is written as CH4+½O2→CO+2H2, where a preferable temperature for the partial oxidation is 250° C. or above.
For the conventional reformers of external heating type and internal heating type, however, the temperature of the heating section becomes a higher temperature than the reforming temperature level of about 700° C., reaching to, for example, as high as 1,000° C. Accordingly the conventional reformers have problems of large energy loss caused by radiation, and generation of high temperature deterioration of the members structuring the reformer, leading to a short life.
As an improved model of the internal heating type, Japanese Patent Laid-Open No. 2001-192201 proposes a reforming apparatus of self-oxidation internal heating type. Although the conventional understanding was that the functions of steam reforming catalyst are hindered under the presence of oxygen, the related art proposed in the patent publication solved the problem by the coexistence of an oxidation catalyst, thus allowed the steam reforming catalyst to effectively maintain the inherent functions thereof even under the presence of oxygen.
The improved technology proposed in the above related art conducts both the heat generation by the oxidation reaction and the steam reforming reaction simultaneously in a mixed catalyst bed structured by an oxidation catalyst and a steam reforming catalyst, respectively. That is, by the coexistence of the oxidation exothermic bed and the steam reforming reaction (endothermic reaction) bed, the temperature of the heating section and the temperature of the heat-absorbing section can be maintained equivalently. Furthermore, the disclosure described that the temperature of structural members including the catalyst can be controlled to a specified reforming temperature or below, for example at near 700° C., thereby making it possible to prolong the life of the structural members. In addition, the apparatus has a function to effectively recover the heat inside the reforming apparatus so that a high reforming efficiency is attained.
According to the reforming apparatus of the self-oxidation internal heating type of the above related art, water supplied from a water feed pump is heated by heat exchange with the reformed gas in a cooler, and further is heat-exchanged in the reforming apparatus to generate steam. Thus generated steam is then mixed with a raw material gas in a mixer.
For the type to generate steam by exchanging heat between water and reformed gas or the like, however, the quantity of generated steam depends on the flow rate and temperature of the reformed gas, thus accurate control of necessary quantity of the generating steam is difficult. In addition, that type has problems of securing space to install a relatively large heat exchange section for generating steam in the reformer, and of complex structure of the reformer.
Although the hydrogen-rich reformed gas generated in the reformer can be used as the fuel for fuel cells, as described before, the structure of above related art cannot reuse the anode flue gas of the fuel cell as the fuel for generating steam.
For the case that the anode flue gas of the fuel cells is combusted to generate steam, the fuel is required to be supplied to a combustor by pressurizing the fuel using, for example, a booster pump. That type of structure makes the system complex and the weight larger. The increased weight is a drawback particularly in a reformer to supply the reformed gas to fuel cells for vehicle.
According to the above related art, the reformer, the mixer, the steam supply system, and the like are fabricated separately, and the total system is structured by connecting between respective devices with conduits. Accordingly, for small systems ranging from 1 kW to 10 kW class, the radiation loss from the conduits connecting individual constructing devices becomes large, which reduces the system efficiency.
In this regard, the present invention aims to solve the problems of above-disclosed self-oxidation internal heating type reformer. An object of the present invention is to provide a novel self-oxidation internal heating steam reforming system that solves the problems.
Another object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which generates steam for reforming at a high efficiency using a compact steam generator.
A further object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which generates a raw material-steam mixture to be supplied to a reformer at a high efficiency without applying a special power unit.
A still another object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which generates steam for use in reforming at a high efficiency by supplying a fuel-air mixture to the steam generator.
A still further object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which attains a high system thermal efficiency.
A more object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, having a reformer of compactness and high reforming efficiency.
A still more object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which recycles the anode flue gas containing hydrogen discharged from a fuel cell as the raw material for reforming when the generated reformed gas is supplied to the fuel cell.
A furthermore object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which efficiently recovers the heat from surplus steam, when it is generated.
A still furthermore object of the present invention is to provide a system, in a self-oxidation internal heating steam reforming system, which generates a reformed gas containing decreased quantity of CO.